Enrollment in the study ""Use of Inhaled Nitric Oxide in the Neonate-Evaluation of Benefit and Toxicity"" began in July of 1999, and to date 7 infants have been enrolled. All infants had persistent pulmonary hypertension as a result of medical illnesses, such as sepsis or meconium aspiration syndrome. Six infants had an initial beneficial response to nitric oxide at 20 ppm after 1 hour (median change in PaO2 +136mmHg, range 12 to +329mmHg). Among the six neonates with respiratory failure and PPHN who initially improved with iNO, 4 survived and did not require ECMO. One infant had a transient increase in PaO2, but ultimately went on to require ECMO 10 hours later. This infant survived. A second infant was a 31 week gestation preterm infant transferred to CHOP on day of life 7 with severe respiratory failure requiring high frequency ventilation, presumed sepsis with hypotension requiring dopamine, and a patent ductus arteriosus. Echocardiogram on arrival at CHOP showed pulmonary hypertension with right to left shunting through the ductus arteriosus. The infant had an initial improvement in PaO2 with iNO from 47mmHg to 263mmHg; however the infant died 1 day later of severe hypotension and respiratory failure. Because of prematurity, this infant was not a candidate for ECMO. The one infant who did not have an improvement in PaO2 with iNO, went on to require ECMO but ultimately survived.